Effects of survivin antagonists on growth of established tumors and B7-1 immunogene therapy

Background. Survivin, a member of the inhibitor of apoptosis (IAP) protein family, is detectable in most types of cancer, and its presence is associat ed with a poor prognosis. We determined the effects of gene-based therapies that inhibit survivin function in a mouse tumor model. Methods: Using five to six mice per treatment group, we injected tumors derived from mouse EL- 4 thymic lymphoma cells with plasmids encoding antisense survivin, a domina nt-negative mutant survivin, and the T-cell costimulator B7-1. Expression o f endogenous survivin and the proteins encoded by the injected plasmids wer e examined by immunohistochemical staining of tumor sections and by western blot and flow cytometry analyses of isolated tumor cells. Tumor growth, th e generation of antitumor cytotoxic T-lymphocyte (CTL) activity, apoptosis, and the contribution of leukocyte subsets to antitumor activity were measu red. All statistical tests were two-sided. Results: Large (1.0-cm. diameter ) tumors had approximately 10-fold more survivin than small (0.2-cm. diamet er) tumors. At 28 days after injection, antisense and dominant-negative mut ant survivin plasmids statistically significantly inhibited the growth of b oth small (P =.006 and P =.0018, respectively) and large (P < .001 for both plasmids) EL-4 tumors compared with tumors injected with empty plasmid. Th e growth of large tumors was further inhibited by intratumoral injection wi th antisense survivin and B74 (P =.004); thus, inhibition of survivin expre ssion renders large tumors susceptible to B7-1-mediated immunotherapy. Mice whose tumors were completely eradicated by injection of B7-1 remained tumo r free for 26 days after re-injection with EL-4 cells (when the experiment ended). Compared with tumors injected with empty plasmid, tumors injected w ith survivin-based plasmids had increased apoptosis, and animals bearing su ch tumors generated more antitumor CTLs. Conclusion: Intratumoral injection of plasmids that block survivin expression and stimulate the generation of tumor-specific CTLs may be beneficial for the treatment of large lymphomas .